Interfacial stress rheometer

Fig. 8 Design of an interfacial stress rheometer. Here a magnetized rod is subjected to an oscillatory force generated by the Helmholtz coils. The motion of the rod is detected using a microscope and photodiode array. Differences between the applied force and resulting phase and magnitude of the displacement give information on the viscoelastic properties of the monolayer. Both the storage modulus G and the loss modulus G" can be determined [2,21] (reproduced with permission from the American Chemical Society)...

Naumann CA, Brooks CF, Fuller GG, Knoll W, Frank CW (1999) Viscoelastic properties of lipopolymers at the air-water interface a combined interfacial stress rheometer and film balance study. Langmuir 15 7752-7761... 

Brooks CF, Fuller GG, Frank CW, Roberston CR (1999) An interfacial stress rheometer to study rheological transitions in monolayers at the air-water interface. Langmuir 15 2450-2459... 

They adapted an interfacial shear rheometer (plate/ rod) to measure the shear viscoelasticity of the system with and without dispersant. At an applied shear stress, creep curves for the system were monitored. There were no instantaneous elasticity and viscosity for the Kuwait and Tia Juana crudes with and wifliout dispersant. They attributed this to a network structure of flocculated asphaltenes in the films. They found that there was some dilatancy in their crude oil films, described as a stick/slip flow in their flow curves. However, fliis flow was attributed to thick films of asphaltene particles building up at the interface. Lfsing creep measurements, they examined a model system of as-phaltenes/n-heptane/toluene. They found a retarded elastic deformation, which was different from the response of the crade oils. This suggested to fliem that there was a different type of interfaeial slrueture formed with the model oil, and this may be attributed to die solveney of the medium and not to die lower asphaltenes eontent in the model system. 

Wasan and his research group focused on the field of interfacial rheology during the past three decades [15]. They developed novel instruments, such as oscillatory deep-channel interfacial viscometer [20,21,28] and biconical bob oscillatory interfacial rheometer [29] for interfacial shear measurement and the maximum bubble-pressure method [15,29,30] and the controlled drop tensiometer [1,31] for interfacial dilatational measurement, to resolve complex interfacial flow behavior in dynamic stress conditions [1,15,27,32-35]. Their research has clearly demonstrated the importance of interfacial rheology in the coalescence process of emulsions and foams. In connection with the maximum bubble-pressure method, it has been used in the BLM system to access the properties of lipid bilayers formed from a variety of surfactants [17,28,36]. 

Biswas and Haydon [20] found some correlation between the viscoelastic properties of protein (albumin or arabinic acid) films at the 0/W interface and the stability of emulsion drops against coalescence. Viscoelastic measurements were carried out using creep and stress relaxation measurements (using a specially designed interfacial rheometer). A constant torque or stress a (mN m ) was applied and the deformation y was measured as a function of time for 30 minutes. After this period the torque was removed and y (which changes sign) was measured as a function of time to obtain the recovery curve. The results are illustrated in Fig. 5.18. From the creep curves one... 

GT Shahin. The Stress Deformation Interfacial Rheometer. PhD thesis. University of Pennsylvania, State College, PA, 1986. 

The complex rheology of polyblends, alloys, and filled plastics can be better understood when both capillary viscometer and stress controlled rotational rheometer are utilized. The capillary viscometer can provide more information about the effects of process history on the macrorheology of these kinds of plastics. This information is complimented with stress controlled rotational rheometry which is more suited for studying the microstructure. Very usefid is the yield stress data which relates to the interfacial bond strength between the clustered particles and the continuous matrix. 

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